Photonic semiconductor devices fall into three categories: devices that convert electrical energy into optical radiation (e.g., light emitting diodes and laser diodes); devices that detect optical signals (e.g., photodetectors); and devices that convert optical radiation into electrical energy (e.g., photovoltaic devices and solar cells). Although all three kinds of devices have useful applications, the light emitting diode may be the most commonly recognized because of its application to various consumer products and applications.
Light emitting devices (e.g., light emitting diodes and laser diodes), herein referred to as LEDs, are photonic, p-n junction semiconductor devices that convert electrical power into emitted light. Perhaps most commonly, LEDs form the light source in the visible portion of the electromagnetic spectrum for various signals, indicators, gauges, and displays used in many consumer products (e.g., audio systems, automobiles, household electronics, and computer systems). LEDs are desirable as light output devices because of their generally long lifetime, their low power requirements, and their high reliability.
Those of ordinary skill in this art will recognize that the abbreviation “LED” is also often used to describe the more specific category of light-emitting diodes. As used herein, the particular meaning will generally clear from the context.
Despite widespread use, LEDs are somewhat functionally constrained, because the color that a given LED can produce is limited by the nature of the semiconductor material used to fabricate the LED. As well known to those of ordinary skill in this and related arts, the process by which light is generated in an LED is referred to as “electroluminescence,” which refers to the generation of light by passing an electric current through a material under an applied voltage. Any particular composition that produces electroluminescent light tends to do so over a relatively narrow range of wavelengths.
The wavelength of light (i.e., the color) that can be emitted by a given LED material is limited by the physical characteristics of that material, specifically its bandgap energy. Bandgap energy is the amount of energy that separates a lower-energy valence band and a higher energy conduction band in a semiconductor. The bands are energy states in which carriers (i.e., electrons or holes) can reside in accordance with well-known principles of quantum mechanics. The “bandgap” is a range of energy between the conduction and valence bands that are forbidden to the carriers (i.e., the carriers cannot exist in these energy states). Under certain circumstances, when an electron from the conduction band crosses the bandgap and recombines with a hole in the valence band, a quantum of energy is released in the form of light. The frequency of the emitted light is proportional to the energy released during the recombination. In other words, the frequency of electromagnetic radiation (i.e., the color) that can be produced by a given semiconductor material is a function of the material's bandgap energy.
In this regard, narrower bandgaps produce lower energy, longer wavelength photons. Conversely, wider bandgap materials produce higher energy, shorter wavelength photons. Blue light has a shorter wavelength—and thus a higher frequency—than the other colors in the visible spectrum. Consequently, blue light must be produced from transitions that are greater in energy than those transitions that produce green, yellow, orange, or red light. Producing photons that have wavelengths in the blue or ultraviolet portions of the electromagnetic spectrum requires semiconductor materials that have relatively large bandgaps.
The entire visible spectrum runs from the violet at or about 390 nanometers to the red at about 780 nanometers. In turn, the blue portion of the visible spectrum can be considered to extend between the wavelengths of about 425 and 480 nanometers. The wavelengths of about 425 nanometers (near violet) and 480 nanometers (near green) in turn represent energy transitions of about 2.9 eV and about 2.6 eV, respectively. Accordingly, only a material with a bandgap of at least about 2.6 eV can produce blue light.
Shorter wavelength devices offer a number of advantages in addition to color. In particular, when used in optical storage and memory devices, such as CD-ROM optical disks, shorter wavelengths enable such storage devices to hold significantly more information. For example, an optical device storing information using blue light can hold substantially more information in the same space as one using red light.
The basic mechanisms by which light-emitting diodes operate are well understood in this art and are set forth, for example, by Sze, Physics of Semiconductor Devices, 2d Edition (1981) at pages 681-703.
The common assignee of the present patent application was the first in this field to successfully develop commercially viable LEDs that emitted light in the blue color spectrum and that were available in commercial quantities. These LEDs were formed in silicon carbide, a wide-bandgap semiconductor material. Examples of such blue LEDs are described in U.S. Pat. Nos. 4,918,497 and 5,027,168 to Edmond each titled “Blue Light Emitting Diode Formed in Silicon Carbide.”
In addition to silicon carbide, candidate materials for blue light emitting devices are gallium nitride (GaN) and its associated Group III (i.e., Group III of the periodic table) nitride compounds such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), and in some circumstances aluminum indium gallium nitride (AlInGaN). These materials are particularly attractive because they offer direct energy transitions with bandgaps between about 1.9 to about 6.2 eV at room temperature. More common semiconductor materials such as silicon, gallium phosphide, or gallium arsenide are unsuitable for producing blue light because their bandgaps are approximately 2.26 eV or less, and in the case of silicon, are indirect semiconductors and inefficient light emitters.
As known to those familiar with LEDs and electronic transitions, a direct transition occurs in a semiconductor when the valence band maxima and the conduction band minima have the same momentum state. This means that crystal momentum is readily conserved during recombination of electrons and holes so that the energy produced by the transition can go predominantly and efficiently into the photon, (i.e., to produce light rather than heat). When the conduction band minimum and valence band maximum do not have the same momentum state, a phonon (i.e., a quantum of vibrational energy) is required to conserve crystal momentum and the transition is called “indirect.” The necessity of a third particle, the phonon, makes indirect radiative transitions less likely, thereby reducing the light emitting efficiency of the device.
Generally speaking, an LED formed in a direct bandgap material will perform more efficiently than one formed in an indirect bandgap material. Therefore, the direct transition characteristics of Group III nitrides offer the potential for brighter and more efficient emissions—and thus brighter and more efficient LEDs—than do the emissions from indirect materials such as silicon carbide. Accordingly, much interest in the last decade has also focused on producing light emitting diodes in gallium nitride and related Group III nitrides.
Although Group III nitrides offer a direct transition over a wide bandgap energy range, the material presents a particular set of technical manufacturing problems. In particular, no commercially-viable technique has yet emerged for producing bulk single crystals of gallium nitride (GaN) that are capable of functioning as appropriate substrates for the gallium nitride epitaxial layers on which photonic devices would be formed.
All semiconductor devices require some kind of structural substrate. Typically, a substrate formed of the same material as the active region offers significant advantages, particularly in crystal growth and lattice matching. Because gallium nitride has yet to be formed in such bulk crystals, gallium nitride photonic devices must be formed in epitaxial layers on non-GaN substrates.
Recent work in the field of Group III nitride substrates includes copending and commonly assigned U.S. Pat. No. 6,296,956, issued Oct. 2, 2001 for “Growth of Bulk Single Crystals of Aluminum Nitride;” U.S. Pat. No. 6,066,205 issued May 23, 2000 for “Growth of Bulk Single Crystals of Aluminum Nitride from a Melt;” U.S. Pat. No. 6,045,612 issued Apr. 4, 2000 for “Growth of Bulk Single Crystals of Aluminum Nitride;” and U.S. Pat. No. 6,086,072 issued Jul. 11, 2000 for “Growth of Bulk Single Crystals of Aluminum Nitride: Silicon Carbide Alloys.”
Using different substrates, however, causes an additional set of problems, mostly in the area of crystal lattice matching. In nearly all cases, different materials have different crystal lattice parameters. As a result, when a gallium nitride epitaxial layer is grown on a different substrate, some crystal lattice mismatching and thermal expansion coefficient mismatching will occur. The resulting epitaxial layer is referred to as being “strained” by this mismatch. Crystal lattice mismatches, and the strain they produce, introduce the potential for crystal defects. This, in turn, affects the electronic characteristics of the crystals and the junctions, and thus tends to degrade the performance of the photonic device. These kinds of defects are even more problematic in high power structures.
In early Group III nitride LEDs, the most common substrate for gallium nitride devices was sapphire (i.e., aluminum oxide Al2O3). Certain contemporary Group III nitride devices continue to use it.
Sapphire is optically transparent in the visible and ultraviolet ranges, but has a crystal lattice mismatch with gallium nitride of about 16 percent. Furthermore, sapphire is insulating rather than conductive, and is unsuitable for conductivity doping. Consequently, the electric current that must be passed through an LED to generate the light emission cannot be directed through a sapphire substrate. Thus, other types of connections to the LED must be made.
In general, LEDs with vertical geometry use conductive substrates so that ohmic contacts can be placed at opposite ends of the device. Such vertical LEDs are preferred for a number of reasons, including their easier manufacture and more simple incorporation into end-use devices than non-vertical devices. In the absence of a conductive substrate, however, vertical devices cannot be formed.
In contrast with sapphire, Gallium nitride only has a lattice mismatch of about 2.4 percent with aluminum nitride (AlN) and mismatch of about 3.5 percent with silicon carbide. Silicon carbide has a somewhat lesser mismatch of only about 1 percent with aluminum nitride.
Accordingly, the assignee of the present invention has developed the use of silicon carbide substrates for gallium nitride and other Group III devices as a means of solving the conductivity problems of sapphire as a substrate. Because silicon carbide can be doped conductively, vertical LEDs can be formed. As noted, a vertical structure facilitates both the manufacture of LEDs and their incorporation into circuits and end-use devices.
As known to those familiar with Group III nitrides, their properties differ based on the identity and mole fraction of the present Group III elements (e.g., gallium, aluminum, and indium). Increasing the mole fraction of aluminum tends to increase the bandgap, while decreasing the amount of aluminum tends to increase the refractive index. Similarly, a larger proportion of indium will decrease the bandgap of the material, thus permitting the bandgap to be adjusted or “tuned” to produce photons of desired frequencies. This, however, tends to reduce the chemical and physical stability of the crystal. Other effects based on mole fraction include changes in crystal lattice spacing resulting in strain effects.
As further known to those familiar with light emitting diodes, appropriate ohmic contacts are required to both the p and n portions of the device. In turn, the theoretical and practical quality of an ohmic contact partly depends on the particular semiconductor, the particular metal, and the manner in which the contact is fabricated. Accordingly, the task of providing ohmic contacts to Group III nitride photonic devices has been, and continues to be, an area of specific interest and need. Exemplary, but neither comprehensive nor limiting, discussions are set forth in, for example, J. Chan et al., Metallization of GaN Thin Films Prepared by Ion Beam Assisted Molecular Beam Epitaxy, 339 Mat. Res. Soc. Proc. p.223 (1994); M. Lin et al., Low Resistance Ohmic Contacts on Wide Band-Gap GaN, Appl. Phys. Lett. 64(8), 21 Feb. 1994; and J. Foresi and T. Moustakas, Metal Contacts to Gallium Nitride, Appl. Phys. Lett. 62(22), 31 May 1993. Recent U.S. Patents on the subject include U.S. Pat. Nos. 6,121,127, 6,046,464 and 5,670,798. Again, these are exemplary of the interest in the field rather than a comprehensive or limiting collection of the relevant art.
In particular, better ohmic contacts to p-type Group III nitrides would desirably have less contact resistivity. Additionally, a desired ohmic contact to p-type nitrides would increase the injection efficiency of holes. In particular the generally wide bandgap of AlGaN tends to produce relatively poor ohmic contacts to p-type material.
Accordingly, a need and interest still exist for devices that incorporate vertical geometry, that take advantage of the characteristics that result when the proportions of indium, aluminum, and gallium are desirably adjusted in the active layers, cladding layers, and buffer layers of Group III nitride photonic devices, and that incorporate ohmic contacts that enhance the performance of the devices.